The present invention relates to a method and apparatus for an oscillator. In particular, the present invention relates to an oscillator that includes a variable capacitor and is insensitive to common-mode noise.
A variable capacitor is useful in tuned electronic circuits. A control signal determines the amount of capacitance that is provided by the variable capacitor. The variable capacitor can be used to make an electronic circuit electrically tunable over a range of frequency. One type of variable capacitor device is a varactor.
A varactor is a nonlinear device that provides a capacitance that varies responsive to an applied control voltage. A diode-type of device is often used as a varactor (hereinafter referred to as a varactor diode). Varactor diodes are operated in a reverse-bias operating mode. When the diode is reverse-biased, the capacitance of the pn junction decreases with an increasing reverse voltage. Although any diode can be used as a varactor, diodes are often specifically designed for this variable capacitance characteristic. A transistor can also be operated as a varactor. By reverse biasing a junction of a transistor, the capacitance of the junction decreases as with the increasing reverse voltage.
Varactors are used in analog circuits that require frequency tuning. Oscillators often use varactors to electrically tune the frequency of oscillation. Electrically tunable oscillators are useful in electronic systems such as frequency modulators and phase-locked loops. LC-type oscillators use an inductor and a capacitor to determine the frequency of oscillation. LC oscillators may be active or passive. Active LC oscillators provide for gain at the oscillator""s resonant frequency. LC oscillators may be arranged in a variety of configurations in order to produce an oscillating signal.
Briefly stated, a differentially controlled variable capacitor includes two variable capacitors that are arranged effectively in parallel and receive differential input. Each variable capacitor can be any device that operates as a variable capacitance, such as a Bipolar device, a MOS device, a MOS capacitor, a reverse-biased diode, or a varactor. The variable capacitors are coupled to a common node that is biased to a potential. For diode-type variable capacitors, the bias potential should ensure that the variable capacitors continue to operate as capacitors by preventing the diode device from becoming forward-biased. The differentially controlled variable capacitors are useful as tuning elements in circuits that require frequency control. A noise signal may be injected into the control signal of the variable capacitors. The effective parallel arrangement of the variable capacitors allows differential control of the effective capacitance value such that the noise signal does not alter the effective capacitance value. The differentially controlled variable capacitor is useful in circuits such as oscillators, tuned filters, and other electronic circuits that require a tuned frequency or tuned capacitance.
In accordance with one aspect of the present invention, an electronic circuit with at least one differentially controlled variable capacitance, comprises a differentially controlled variable capacitance that has a differential input and is coupled to a common node that is biased to a potential. Also a differential control signal is coupled to the differential input such that the differential control signal changes a capacitance value associated with the differentially controlled variable capacitance.
In accordance with another aspect of the present invention, a method for tuning an oscillation frequency in an oscillator, comprises coupling a first variable capacitor to a common node in the oscillator, the first variable capacitor having a first associated value that is responsive to a first control signal. Also, coupling a second variable capacitor to the common node in the oscillator, the second variable capacitor having a second associated value that is responsive to a second control signal. Then adjusting at least one of the first and second control signals such that the first and second variable capacitors provide for tuning of the oscillation frequency that minimizes variations in the oscillation frequency due to common mode noise on the first and second control lines.
In accordance with yet another aspect of the present invention, an apparatus for minimizing noise effects in an oscillator with a variable oscillation frequency, comprises: a first variable capacitance means is coupled to a common node in the oscillator; a second variable capacitance means is coupled to the common node in the oscillator; a first means for controlling the first variable capacitance means; a second means for controlling the second variable capacitance means; and means for varying the oscillation frequency with the first means for controlling and the second means for controlling such that the variable oscillation frequency is changed by the first and second variable capacitance means, whereby variations in the variable oscillation frequency from noise effects are minimized by the arrangement of the first and second variable capacitance means.